1. Field of the Invention
The present invention relates to a two-port non-reciprocal circuit device, such as an isolator used in a microwave band, a composite electronic component, and a communication apparatus.
2. Description of the Related Art
Two-port non-reciprocal circuit devices disclosed in U.S. Pat. No. 4,016,510 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2002-299916 (Patent Document 2) are known. In the two-port non-reciprocal circuit device according to Patent Document 1, a first central electrode and a second central electrode are arranged on the upper surface of a ferrite member such that the electrodes cross each other and are electrically insulated from each other. One end of each of the first and second central electrodes is grounded, and the other ends thereof are electrically connected to an input terminal and an output terminal, respectively. Also, a resistor is electrically connected between the other end of the first central electrode and the other end of the second central electrode. Further, matching capacitors are electrically connected between the other ends of the first and second central electrodes and ground. Each of the input and output terminals is an unbalanced terminal.
In the two-port non-reciprocal circuit device shown in FIG. 11 of Patent Document 2, one end of a first central electrode in the input side is grounded and the other end thereof is electrically connected to an unbalanced input terminal. A matching capacitor is electrically connected between the other end of the first central electrode and ground. Both ends of a second central electrode in the output side are electrically connected to a balanced output terminal through matching capacitors. Also, a resistor is electrically connected between the other end of the first central electrode and the other end of the second central electrode. Further, a matching capacitor is electrically connected between both ends of the second central electrode.
In the two-port non-reciprocal circuit device of the Patent Document 1, however, both input and output terminals are unbalanced terminals, and thus cannot be connected to a balanced circuit. In order to connect the two-port non-reciprocal circuit device to the balanced circuit, a balanced-to-unbalanced transformer (balun) has to be provided to the input/output sides of the non-reciprocal circuit device. In this way, when the balun and the non-reciprocal circuit device are used together, the size and cost of the device increase, the structure is complicated, and reliability decreases.
Also, in the two-port non-reciprocal circuit device of Patent Document 2, only one of the two balanced output terminals is electrically connected to an unbalanced input terminal via a resistor. Therefore, it causes imbalance between the two balanced output terminals. Accordingly, the common mode rejection ratio of the non-reciprocal circuit device decreases, and thus the amount of signals which are input to the two balanced terminals in a common mode and which are output increases. As a result, undesired waves other than necessary signal waves pass through the non-reciprocal circuit device disadvantageously.